Ozone peaks associated with a subtropical tropopause fold and with the trade wind inversion: A case study from the airborne campaign TROPOZ II over the Caribbean in winter

Abstract

Aircraft measurements of ozone, methane, carbon monoxide, relative humidity, and equivalent potential temperature were performed during the TROPOZ II campaign. During the aircraft descent down to Pointe-h-Pitre (16.3øN, 61.5øW), at 2100 UTC on January 12, 1991, two ozone peaks (75 ppb) are observed, one at an altitude of 7.5 km and the other at 3.0 km. A physicochemical interpretation for each ozone peak is proposed in connection with the meteorological context, using radiosounding data, total ozone content from TOMS/NIMBUS 7 and diagnoses issued from analyses by the European Centre for Medium-Range Weather Forecasts, Reading, England. The stratospheric origin of the 7.5-km ozone peak is inferred from negative correlations between ozone and its precursors and from diagnoses based on potential vorticity and ageostrophic circulations depicting the structure of the tropopause fold embedded in the subtropical jet front system. Using an appropriate method to isolate cross-and along-front ageostrophic circulations, we show that much of the observed structure of the tropopause fold can be ascribed to transverse and vertical circulations associated with the irrotational part of the flow. Though the downward extent of the subtropical tropopause fold (400 hPa) is restricted in comparison with typical extratropical tropopause ones (700 hPa), the present results suggest that subtropical tropopause folds may significantly contribute to the global stratosphere-troposphere ozone exchange. The origin of the 3.0-km ozone peak trapped just below the trade wind inversion cannot be ascribed precisely. Analogies with other measurements of dust and aerosols transported over the Atlantic or Pacific in the summer season are discussed. Various possibilities are examined:(1) an earlier stratospheric intrusion event, (2) long-range transport by the trade winds of biomass burning species emitted over West Africa, and (3) fast photochemical ozone formation occurring just below the trade wind inversion within already polluted air parcels originating from remote regions (United States and Gulf of Mexico) after eastward and southward transport around the western Atlantic anticyclone.

Spatial and temporal variation of MOPITT CO in Africa and South America: A comparison with SHADOZ ozone and MODIS aerosol

“…This suggests, that biomass burning is not the source of tropospheric ozone at these stations at these times. Recent studies have shown that Stratospheric‐Tropospheric Exchange (STE) can take place near the edge of the tropical zones [ Folkins and Appenzeller , 1996; Gouget et al , 1996; Baray et al , 1998; Zachariasse et al , 2000]. In particular STE has been invoked in recent years at Reunion to explain observations of tropospheric ozone increases [ Randriambelo et al , 1999, 2000; Baray et al , 2000].…”

Influence of corona discharge on the ozone budget in the tropical free troposphere: a case study of deep convection during GABRIEL

“…The BL is capped by an inversion layer at 3.0 km, which was observed on several flights during the campaign. This so-called trade wind inversion has been reported in different studies on tropical regions and is known to act as an effective transport barrier so that trace species can accumulate in the BL (Schubert et al, 1995;Gouget et al, 1996;Andreae et al, 2001). The vertical profile of CO shows a high mixing ratio of (131.7 ± 10.1) ppbv in the lower troposphere and a slightly lower value of (116.0 ± 4.3) ppbv in the outflow region.…”

Characteristics of tropopause folds over Arctic latitudes

“…In all of these studies, the tropopause folds have been observed at mid latitudes. However, tropopause folding also occurs in the baroclinic zones beneath subtropical [ Gouget et al , 1996] and arctic [ Shapiro et al , 1984, 1987] jet streams. In fact recent work [ Sprenger et al , 2003] indicates the occurrence of more folds in the subtropics than at mid latitude.…”